Ultrahigh-frequency electromag-netic radiation heating method and apparatus



March 24, 1953 G. w. SCHROEDER 2,632,838

ULTRAHIGH-FREQUENCY ELECTROMAGNETIC RADIATION HEATING METHOD AND APPARATUS Filed March 4, 1948 INVENTOR.

George W. Schroeder B Y M, O um MSW Aflys Patented Mar. 24, 1953 ULTRAHIGH-FREQUENCY ELECTROMAG- NETIC RADIATION HEATING METHOD AND APPARATUS George W. Schroeder, McHenry, 111., assignor to General Electric Company, a corporation of New York Application March 4, 1948, Serial No. 12,998

9 Claims.

1 A The present invention relates to ultra-high frequency electromagnetic radiation heating methods and apparatus.

In the processing of semi-conducting materials, such as foods, cellulose products, organic substances, etc., it has been proposed to employ ultra-high frequency electromagnetic heating as distinguished from low radio frequency induction heating and high radio frequency dielectric heating, both positioned lower in the spectrum, and from infra-red radiation heating, positioned higher in the spectrum. The utilization in a cavity of ultra-high frequency electromagnetic waves of the order of 1000 megacycles, waves of a length normally measured in centimeters, in the heating of semi-conducting material arranged in the cavity is very advantageous in certain respects in that the inner portion of the material disposed adjacent to the center thereof and the outer portion of the material disposed adjacent to the surface thereof are both heated simultaneously and very quickly to a normal cooking temperature. However, since centimeter electromagnetic standing waves are produced in the cavity the energies in the electric field and in the magnetic field are not uniformly distributed throughout the cavity, whereby localized heating effects are produced in the mass of semi-conducting material arranged in the cavity resulting in uneven heating or cooking of the different portions thereof.

Accordingly, it is a general object ofthe present invention to provide an improved method of heating semi-conducting material in a cavity employing ultra-high frequency electromagnetic radiation in such a manner that the heating effects caused in the material are distributed substantially uniformly therethrough.

Another object of the invention is to provide an ultra-high frequency heater including a cavity in which ultra-high frequency electromagnetic radiation is produced and embodying a movable material supporting element, whereby the heating effects caused in the supported material are distributed substantially uniformly therethrough.

A further object of the invention is to provide anultra-high frequency heater of the character noted that is of improved and exceedingly simple construction and arrangement.

Further features of the invention pertain to the particular arrangement of the steps of the method and of the elements of the ultra-high frequency heater, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following. specification taken in connection with the accompanying drawing, in which Figure 1 is a plan view of an ultra-high frequency heater embodying the present invention, and in conjunction with which the method of the present invention may be carried out, the cover for the cavity in the heater being illustrated in its removed or openposition; and Fig. 2 is a vertical sectional view of the heater shown in Fig. 1, the cover for the cavity in the heater being illustrated in its normal or closed position.

Referring now to the drawing, the ultra-high frequency heat-er I i! there illustrated and embodying the features of the present invention, comprises a base II and an upstanding metal shell I2 provided with a substantially flat rectangular top wall 13, the base H and the shell 12 cooperating to define a chamber M. The top wall l3 has a relatively large annular opening i5 formed therein into which a metal Vessel I6 is removably arranged within the chamber M. The vessel It comprises a cylindrical side wall I! and a flat bottom wall It, the upper portion of the side wall l1 terminating in an annular surrounding flange l9 carrying an annular fixture 20. The annular fixture 2i) is arranged directly in the opening l5 and is provided with an; out* wardly directed annular flange 2! projecting over the adjacent annular portion of the top wall 13 in order to support the vessel It. Accordingly, the vessel 26 comprises an upstanding substantially cylindrical cavity 22 and is provided with an open top. Finally, the vessel It is provided with a detachable metal cover 23 having a centrally located handle 24, the cover 23 being outwardly dished and of substantially disk-like configuration.

The cover 23 may be detachably secured in place upon the open top of the vessel It by a bayonet joint arrangement including a plurality of substantially equally angularly disposed slots 25 formed in the peripheral edge of the cover 23 and a cooperating plurality of substantially equally angularly disposed and radially directed tabs 26 carried by the fixture 26; Finally, an annular gasket 21 formed of compressible metal braid or the like is arranged between the upper portion of the annular flange l9 and the adjacent lower peripheral portion of the cover 23 in order to provide a seal therebetween. Of course, it will be understood that the cover 23 may be placed upon the open top of the vessel is by causing the slots 25 to register with the tabs 25. The cover 23 is then urged downwardly to compress the gasket 21, and is then rotated in order to cause the slots 25 to move out of alignment with respect to the tabs 26 so that the tabs 26 engage the associated portions of the cover 23 securely clamping it in place.

Also the heater I comprises an ultra-high frequency generator 28 arranged Within the chamber l4 and supported upon a surrounding metal shield 29, that in turn, is supported mutually by the adjacent wall of the shell [2 and the adjacent side wall I! of the vessel It. More particularly, the side wall I! of the vessel l6 disposed adjacent to the generator 28 is provided with an opening 30 therein communicating with the interior of the shield 29. Further the generator 28 is connected to two antennae 3| projecting through the opening 30 into the cavity 22 formed in the vessel l6 and ultimately connected to the side wall I'I. More particularly, each of the antennae is substantially U-shaped, one terminal thereof being connected to the generator 28 and the other terminal thereof being connected to the side wall I! and the intermediate portion thereof projecting through the opening 30. Specifically the two antennae 3! are arranged substantially in the same vertical plane bisecting the cavity 22 formed in the vessel I6; whereby the two U-shaped turns of the two antennae are in good radiating position with respect to the central portion of the cavity 22. The generator 28 may be of any suitable ultrahigh frequency type producing an alternating voltage between the antennae 3| of a frequency of the order of 1000 megacycles; whereby centimeter electromagnetic waves are produced in the cavity 22. In this example the wave length of the electromagnetic waves produced in the cavity 22 is of the order of 30 centimeters. Accordingly, electromagnetic standing waves are produced in the cavity 22 and comprise electric and magnetic fields penetrating the space in the cavity 22. Since the shield 29 surrounds the generator 23 and is connected electrically both to the metal shell l2 and to the side wall H of the metal vessel l6, and since the vessel [6 is electrically connected through the metal gasket 21 to the metal cover 23, the cavity 22 is completely closed so that the ultra-high frequency electromagnetic radiation does not escape from the generator 23 or from the antennae 3| or from the vessel E6 to the exterior in the vicinity of the heater Iii. Thus it will be understood that the generator 28 will be operated only when the cover 23 is in place upon the vessel [6.

Further the heater l0 comprises an annular material supporting platform or turntable 32 arranged within the cavity 22 and mounted for rotation. More particularly, the platform 32 is rigidly secured to the upper end of a rotatable shaft 33 projecting through an opening 34 formed in the bottom wall I8, which rotatable shaft is provided with bearing structure 35 carried by the bottom wall I8 immediately below the platform 32. The lower end of the rotatable shaft 33 is, in turn, connected to the operating shaft of a synchronous motor driven gear train, indicated at 36, and suitably supported within the chamber M upon the base II. Preferably, the synchronous motor driven gear train 36 constitutes the usual Telechron synchronous motor unit operated by a commercial source of 60 cycle alternating current; whereby the shaft 33 and consequently the platform 32 is rotated about one revolution per minute. In passing it is noted that the platform 32, the shaft 33 and the bearing structure 35 are made of electrical insulating material such as lava which is not heated by the ultra-high frequency electromagnetic radiation in the cavity 22. Also the opening 34 formed in the bottom wall I8 is sufficiently small to prevent the escape of the ultra-high frequency electromagnetic radiation from the cavity 22 to the exterior in the vicinity of the heater 50. Finally, a mass of semi-conducting material 3'! to be heated or cooked is supported by the plat form 32; which material 31 may comprise a food, a cellulose product, an organic substance or the like.

In the operation of the heater it, the mass of material 371 is placed upon the platform 32, the cover 23 is then securely clamped in place upon the vessel it the circuit for operating the synchronous motor unit 36 is closed and operation of the generator 20 is initiated. The antennae 3i radiate ultra-high frequency electromagnetic waves producing centimeter electromagnetic standing waves in the cavity 22 permeating the mass of material 3?.

It has been discovered that the points of maximum energy of the electric and magnetic fields in the cavity 22 occur at a position substantially coincident with the vertical center line of the cavity 22, thereby producing localized heating effects in this portion of the mass of material 31. In order to distribute these heating effects throughout the mass of material 3?, the platform 32 is rotated by the synchronous motor unit 36. Also since these points of maximum electric and magnetic field energies occur adjacent to the vertical center line of the cavity 22, it is very advantageous that the vertical center line of the rotatable platform 32 be positioned offset with respect thereto. Specifically, the vertical center line of the rotatable platform 32 is ofiset somewhat toward the right with respect to the verti cal center line of the cavity 22; whereby a large annular portion of the mass of material 3? is rotated through the points of maximum energy of the electric and magnetic fields of the centimeter electromagnetic standing waves in the cavity 22. This offset arrangement of the center lines mentioned is very advantageous as it facilitates substantial uniform distribution of the localized heating efiects produced in the mass of material 37. Also it has been discoveredthat another localized heating effect is produced in the mass of material 3'! in the immediate vicinity of the antennae 3!; which localized heating effect is perhaps due to the attenuation of the power of the ultra-high frequency electromagnetic radiation as it enters the adjacent side of the mass of material 31 and progresses there' through toward the center and the opposite side thereof. Thus the utilization of the rotating platform 32 also distributes this localized heating effect substantially uniformly through the mass of material 31.

Accordingly, it will be understood that in the event the platform 32 is not rotated localized heating effects are produced in the mass of ma terial 31, which localized heating effects seem to appear at the points of the maximum energy in the electric and magnetic fields of the electromagnetic standing waves in the cavity 22 and also in the immediate vicinity of the antennae 3!. Thus it will be understood that by effecting rotation of the mass of material 31, the different portions thereof are periodically subjected to the localized heating effects mentioned; whereby the overall heating effects produced in the mass of material 3? are substantially uniformly distributed therethrough in order to effect substantially uniform heating or cooking of the entire mass of material 37. In the operation of the heater 19, the entire mass of the material 31 is heated and cooked very quickly, and the portion adjacent to the interior or center thereof and the portion adjacent to the exterior or surface thereof are heated and cooked simultaneously.

After the mass of material 3'! has been heated or cooked the circuit for operating the synchronous motor unit 35 is opened and operation of the generator 28 is arrested. The cover 23 is then removed from the vessel It and the mass of material 3! may be removed through the open top of the cavity 22. In passing it is noted that the mass of material 3'! may be immediately removed from the cavity 22 after it has been heated or cooked even though it is contained in a glass or other insulating dish, since the dish will not be particularly hot as no direct heating thereof takes place. Also another characteristic of ultra-high frequency cooking is that the mass of material 3! is not browned on the outer surface thereof although it is thoroughly and uniformly cooked therethrough. By way of illustration it is noted that the semi-conducting ingredients of a cake or the like weighing several pounds may be thoroughly and adequately cooked in the heater id in a matter of a few minutes; whereby the method and, the heater I!) of the present invention are admirably suited to the mass preparation of bakery or similar goods.

The method of the present invention is especially suited to the heating or cooking of semiconducting materials, such as foods, since such materials cannot readily be heated by low radio frequency induction heating methods because the electrical resistance thereof is too great, and since such materials cannot readily be heated by high radio frequency dielectric heating methods because the dielectric strength thereof is too small. However, when the ultra-high frequency cavity heating method of the present invention is employed, the semi-conducting material being heated exhibit a substantially uniform temperature rise throughout caused by the combined electric and magnetic fields within the cavity and notwithstanding the circumstance that the wave length of the standing waves in the cavity is of the order of the dimensions of the material being heated since the material is moved into a plurality of different positions in the standing waves as it is being heated.

In view of the foregoing, it is apparent that there has been provided an improved method of and heater for carrying out ultra-high frequency heating or cooking of semi-conductors in a thorough, rapid and economical manner.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. The method of heating semi-conducting material arranged in a cavity comprising producing ultra-high frequency electric and magnetic fields having maximum intensities in predetermined positions in the cavity in order to cause heatin effects in the material, and. rotating the material in an offset position with respect to the predetermined positions in the cavity so that different portions of the material are, rotated only periodically through the predetermined position in the cavity in order to distribute the heating effects through the material.

2. The method of heating semi-conducting material arranged in a cavity comprising producing centimeter electromagnetic standing waves having maximum intensities in predetermined positions in the cavity in order to cause heating effects in the material, and moving the material in an offset position with respect to the predetermined positions in the cavity so that different portions of the material are moved only periodically through the predetermined positions in the cavity in order to distribute the heating efiects through the material.

3. An ultra-high frequency heater comprising walls made of electrically conducting material forming a cavity, means for producing ultra-high frequency electric and magnetic fields havin maximum intensities in predetermined positions in said cavity, a rotatable element arranged in said cavity and offset with respect to said predetermined positions therein and adapted to support material in said fields in order to cause heating effects in the supported materials, and means for rotating said element so as to move different portions of the supported material through said predetermined positions in said cavity in order to distribute the heating effects through the supported material.

4. An ultra-high frequency heater comprising walls made of electrically conducting material forming a cavity, means for producing ultra-high frequency electric and magnetic fields having maximum intensities in predetermined positions in said cavity, a rotatable platform arranged in said cavity and offset with respect to said predetermined positions therein and adapted to support material in said fields in order to cause heating effects in the supported material, and means for rotating said platform so as to move different portions of the supported material through said predetermined positions in said cavity in order to distribute the heating effects substantially uniformly through the supported material.

5. An ultra-high frequency heater comprising walls made of electrically conducting material forming a cavity having an opening thereinto, a removable cover made of electrically conducting material adapted to cooperate with said opening in order to close said cavity, a rotatable material supporting platform arranged in said cavity and accessible from the exterior through said opening when said cover is removed, means for producing ultra-high frequency electric and magnetic fields having maximum intensities in predetermined positions in said cavity when said cavity is closed by said cover in order to cause heating effects in material supported by said platform, the center of rotation of said platform being offset with respect to said predetermined positions in said cavity, and means for rotating said platform so as to move different portions of the supported material through said predetermined positions in said cavity in order to distribute the heating effects through the supported material.

6. An ultra-high frequency heater comprising walls made of electrically conducting material forming a cavity, means for producing centimeter electromagnetic standing waves having maximum" intensities in predetermined positions in said cavity, a rotatable element arranged in said cavity and offset with respect to said predetermined positions therein and adapted to support material in said standin waves in order to cause heating effects in the supported material, and means for rotating said element so as to move different portions of the supported material through said predetermined positions in said cavity in order to distribute the heating effects through the supported material.

7. An ultra-high frequency heater comprising walls made of electrically conducting material forming a cavity, means for producing an ultrahigh frequency electromagnetic field in said cavity, said field having a maximum intensity at a predetermined position substantially coincident with the center of said cavity, a turntable mounted for rotation in said cavity about a sub stantially vertical axis offset from the center of said cavity and adapted to support material in said field in order to cause heating effects in the supported material, and means for rotating said turntable so as to move different portions of the supported material through said predetermined position in said cavity in order to distribute the heating effects substantially uniformly through the supported material.

8. An ultra-high frequency heater comprising walls made of electrically conducting material forming a cavity, an element arranged in said cavity and mounted for movement in offset relation to predetermined positions in said cavity and arranged to support semi-conducting material in said cavity, means for moving said element so that different portions of the supported material are moved only periodically through said predetermined positions in said cavity, and means for producing ultra-high frequency electric and magnetic fields in said cavity that have maximum intensities substantially in said predetermined positions so that the heating effects produced in the supported material are substantially uniformly distributed therethrough.

9. A'high-=frequencyheater comprising walls made of electrically conducting material forming a chamber, high frequency supply means connected to said chamber for producing standing electromagnetic field waves in said chamber including electric and magnetic fields having maximum energy at a position substantially coincident with the center line of said chamber, a support in said chamber for the material to be heated mounted for rotation about an axis offset with respect to and substantially parallel with the center line of said chamber so as to pass all parts of the material through substantially the same aggregate electric and magnetic fields thereby to effect uniform heating of the material in said chamber, and driving means connected to rotate said support.

GEORGE W. SCI-IROEDER.

REFERENCES CITED The following references are of record in th file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Marcum et al., Possible Uses of Microwaves for Industrial Heating, Product Engineering, January 1947, pages 137-140.

Marcum et al., Heating with Microwaves, Electronics, March 1947, pp. 82-85. 

